This invention relates to solid-state electronic control systems for furnaces and, more particularly, to such control systems which include a means for heating the water in domestic hot water tanks.
In U.S. Pat. No. 4,907,737, I disclosed an ELECTRONIC CONTROL SYSTEM FOR A GAS-FIRED/HOT AIR FURNACE WITH INDUCED DRAFT BLOWER which regulates heating and cooling of a space and monitors the furnace for excessive temperatures. This furnace control system does not, however, provide means for diverting heated water from the heat exchanger in the furnace to the heating coil of a separate hot water heater.
While it has generally been considered desirable to utilize the heated water from the heat exchanger of a hot air furnace system to heat the water in a domestic hot water heater, this arrangement has certain drawbacks related to the generally, limited heating capacity of the furnace. In this connection, when the water from the heat exchanger of the furnace is directed to the heating coil of the hot water tank, to avoid sending out cool air from the furnace when the space thermostat calls for heat, the comfort fan must be cut "off".
In the past, a circuit for shutting "off" the comfort fan would have generally included a normally "closed" relay switch connected in a series circuit with the fan so that when the hot water tank called for heat, the comfort fan would be cut "off". The coil of the relay is connected to a temperature switch disposed to sense the temperature of the water in the hot water tank and is adapted to open when heat is required. In addition, a temperature switch is provided on the heat exchanger for the furnace so that the comfort fan will not start until the heat exchanger reaches a sufficiently high temperature to heat the furnace air for circulation to the heated space. One of the drawbacks of this circuit was that it required two separate temperature operated switches, and an additional relay with normally closed contacts.
Accordingly, the principal object of this invention is to provide an improved control circuit to be integrated with a solid state furnace control system used in combination with a hot air furnace with a hot water heat exchanger and a hot water supply heater.
Another object of this invention, in an improved control circuit of the above type, is to eliminate the need for a second temperature switch and an additional relay for controlling the operation of the comfort fan.